Have you ever wondered why the mobile phone plug is larger than the average plug?
In fact, strictly speaking, this "plug" is called the switching power adapter, the purpose is to convert the AC power transmitted from the power grid into direct current transmission into electronic devices. Laptop's power adapter is bigger, winter warm hands more convenient ~
Why do you want to change AC power to DC power? Let us start from the beginning.
Electricity 
Electricity is a natural phenomenon. The static or moving charge will produce many interesting physical phenomena, such as the lightning that will be seen in thunderstorms, and the sparks that pop in the winter when they slip off their sweaters. Later, scientists discovered the laws from various electrical effects and invented batteries, generators, and motors.
Do you use DC or AC first?
220 volt/50 Hz AC is the standard for power supply in China, so many people think that household appliances use AC power. As for direct current power, we probably don't think of anything else other than the “DC inverter air conditioner†in the ad.
In fact, DC applications are very extensive and you must have seen and used them. such as:
definition
Direct current refers to the current whose direction does not change with time.
Alternating current refers to the current that changes periodically in size and direction.
Observed from the oscilloscope, one is straight, one is curved...
Anyway, it's all electricity. It's not a big difference.
No~ No~ No~
Let's turn history and talk about gossip!
In 1884, a young man named Tesla took the introduction letter of his former employer and hurriedly boarded a ship bound for the gold rush to the United States. He wants to find his idol, Thomas Edison, and hopes that he can help him complete the invention of the AC system.
Thomas Edison
The letter read: "Dear Edison: I know two great men, one is you and the other is the young man." At the time, Edison was marketing his own DC system to the world and he was not optimistic about AC. With this letter, Tesla still entered Edison's team. Edison promised that if he solved the "DC generator and motor problem", he would pay him a bonus equal to today's one million US dollars. But when Tesla had repaired Edison's machine and asked, "Shall I say a million?†Edison smiled. He said, "Tesla, you don't understand our American humor." Tesla, who had been teased and tossed away, ran out of anger and became an independent portal. From then on, he devoted himself to his AC system.
Tesla and AC motors
At the time, Edison’s DC system required a power station within every square mile, and because the transmission losses were too great and the transmission distance was very limited, Tesla’s AC system used finer wires. Higher, less transmission loss, transmission distance. Obviously, AC has more advantages. What did Edison do at a disadvantage? In order to "black" AC, Edison opened his brain. Bribed a government official and changed the death penalty from a hanged to an AC.
AC electric chair
Even employing elementary school students to electrocute stray cats and dogs by electrocution to promote AC is "very dangerous." Fortunately, Tesla decided that "you are not black and the AC power is there". Through the lighting project of the Colombian Expo, it demonstrated the reliability and safety of AC power. Eventually it won the "current battle" and also returned to its own. clean.
Aerial view of Columbia World's Fair
Since then, AC power has been recognized as a replacement for DC power and has become the mainstream of power supply.
AC, DC, in the end who is better?
With the continuous increase of line voltage, transmission power and transmission distance, DC power has been favored by engineers. Because DC does not need rectification and filtering, there is no phase difference and it is relatively stable. How to boost DC voltage? Simply speaking, the boosting work is handed over to the AC, AC-DC conversion, and so on. Moreover, from the economic point of view, although the DC converter station is more expensive than the AC substation, the DC line is only required to be positive and negative. Lines, three lines of AC lines require three lines, DC lines are cheaper, so the longer the distance, the more suitable for HVDC transmission.
example
At present, the world's highest voltage level transmission project is the DC project - ± 1100 kilowatts of quasi-East - Weinan project. The total length of the project line is 3,324 kilometers, which is longer than the straight line distance from Harbin to Haikou.
In the field of power transmission, the asynchronous connection between two underwater systems and more than 30 kilometers of underwater cables is also a direct current.
Alternating current and direct current have their own characteristics. Some people say, for example, AC power is like high-speed railways, DC power is like air planes, one can stop halfway, and one flies point to point. Which one to use, we must analyze specific issues.
Do household appliances use direct current?
Back in the field of home appliances, DC power was used internally in IT products (eg mobile phones, computers) and most home appliances. Because laptops and mobile phones require light weight, the rectifiers are placed outside, and desktop computers, refrigerators, air conditioners, etc. put the rectifiers inside the appliances.
why? Back to the essence, the direct current is continuously flowing from the positive electrode to the negative electrode, and the alternating current is fluctuating. The electronic components work by recognizing the high and low potentials. For example, a computer has a potential of 1 and no potential is 0. The AC power itself has a potential of zero crossing, so that the electronic components cannot perform correct logic judgment.
Electricity is an ever-evolving complex knowledge. This article obviously cannot explain every situation in detail. However, if the girlfriend asks, "Is there a mobile phone charging plug?" You can take this article calmly and correct it by saying, "My dear, this is called the power adapter." - A suggestion from a single dog.
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